fix(python): expose FTS build memory limits (#3796)

## Summary

- expose `memory_limit` and `num_workers` on the Python FTS
configuration for local builds
- forward both build-only settings to the Lance inverted-index builder
- add an end-to-end regression proving the configured memory budget
reaches the native build

## Root cause

LanceDB 0.26.1 pinned Lance 1.0.1. That Lance version used an FTS
partition-merge path whose retained data made memory grow with merge
progress on very large indexes. Upstream Lance
[#5754](https://github.com/lance-format/lance/pull/5754) changed
partition merging to stream its inputs, reducing peak memory by about
25%. Lance [#6174](https://github.com/lance-format/lance/pull/6174) then
removed the old merge phase, compressed posting lists during
construction, reduced indexing memory by about 60%, and introduced a
total build `memory_limit` for bounded workers.

Current `main` pins Lance 11.0.0-beta.3, which contains those
architectural fixes. This PR does not duplicate or claim the upstream
leak fix; it addresses the remaining Python API gap.

## This repair

LanceDB Python did not expose the native FTS builder resource controls.
`memory_limit` now sets the total local-build budget in MiB, divided
among effective workers, and `num_workers` controls build parallelism.
Both are build-only settings and do not affect remote builds or
persisted index configuration.

## Validation

- `cargo check --quiet --features remote --tests --examples`
- `cargo fmt --all`
- `uv run --project python --extra tests --extra dev ruff check .`
- `uv run --project python --extra tests --extra dev ruff format --check
python/python/lancedb/index.py python/python/tests/test_fts.py`
- `uv run --project python --extra tests pytest python/tests/test_fts.py
-q` (51 passed)

Fixes #2923

<!-- lance-gatekeeper-fix:v1 agent=a1ceedf74531e0212cb6f1ebf9390a26
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/src/index.rs

@@ -42,7 +42,7 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
             "Fm" => Ok(LanceDbIndex::Fm(FmIndexBuilder::default())),
             "FTS" => {
                 let params = source.extract::<FtsParams>()?;
-                let inner_opts = FtsIndexBuilder::default()
+                let mut inner_opts = FtsIndexBuilder::default()
                     .base_tokenizer(params.base_tokenizer)
                     .language(&params.language)
                     .map_err(|_| {
@@ -61,6 +61,12 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                     .ngram_max_length(params.ngram_max_length)
                     .ngram_prefix_only(params.prefix_only)
                     .custom_stop_words(params.custom_stop_words);
+                if let Some(memory_limit) = params.memory_limit {
+                    inner_opts = inner_opts.memory_limit_mb(memory_limit);
+                }
+                if let Some(num_workers) = params.num_workers {
+                    inner_opts = inner_opts.num_workers(num_workers);
+                }
                 let inner_opts = inner_opts
                     .block_size(params.block_size)
                     .map_err(|err| PyValueError::new_err(err.to_string()))?;
@@ -213,6 +219,8 @@ struct FtsParams {
     ngram_max_length: u32,
     prefix_only: bool,
     block_size: usize,
+    memory_limit: Option<u64>,
+    num_workers: Option<usize>,
 }
 
 #[derive(FromPyObject)]
@@ -444,3 +452,51 @@ impl IndexConfig {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pyo3::types::{PyDict, PyDictMethods};
+    use serde_json::json;
+
+    #[test]
+    fn fts_build_controls_are_forwarded() {
+        Python::initialize();
+        Python::attach(|py| {
+            let locals = PyDict::new(py);
+            py.run(
+                c"class FTS:
+    with_position = True
+    base_tokenizer = 'simple'
+    language = 'English'
+    max_token_length = None
+    lower_case = True
+    stem = False
+    remove_stop_words = False
+    custom_stop_words = None
+    ascii_folding = False
+    ngram_min_length = 3
+    ngram_max_length = 3
+    prefix_only = False
+    block_size = 128
+    memory_limit = 2048
+    num_workers = 7
+
+config = FTS()",
+                None,
+                Some(&locals),
+            )
+            .unwrap();
+
+            let config = locals.get_item("config").unwrap().unwrap();
+            let index = extract_index_params(&Some(config)).unwrap();
+            let LanceDbIndex::FTS(params) = index else {
+                panic!("expected FTS index parameters");
+            };
+            let training_json = params.to_training_json().unwrap();
+
+            assert_eq!(training_json.get("memory_limit"), Some(&json!(2048)));
+            assert_eq!(training_json.get("num_workers"), Some(&json!(7)));
+        });
+    }
+}